Recently, as a method to achieve ultra high-speed and large-capacity signal transmission in an optical transmission system, a phase modulation technique such as Differential Quadrature Phase Shift Keying (DQPSK) and a digital coherent technique such as Dual-Polarized Quadrature Phase Shift Keying (DP-QPSK) are under active investigation. In order to seek a further expanded transmission distance, such a method of Alternate Polarization Differential Phase Shift Keying (Apol-DPSK) is also under consideration, which uses orthogonally polarized different waves alternately for symbols to be transmitted.
In optical DPSK and DQPSK methods, a wave detector is utilized in a receiver having a delayed interferometer that detects phase difference between two adjacent symbols of transmission. Accordingly, a transmitter is required to have a circuit called as a differential encoding circuit or a pre-coder that pre-assigns transmission data to phase differences. In a digital coherent method, although the phase of light received is detected by using receiver's local light, its absolute phase at a transmitter is unknown, thus making it unable to reproduce the data. For dealing therewith, a differential encoding may be employed because it is capable of reproducing data even if the initial phase is unknown.
In a conventional Alternate Polarization Differential Phase Shift Keying (Apol-DPSK) method, by using polarized waves orthogonal for every one symbol, withstanding ability is enhanced against signal degradation due to a non-linear effect, such as self-phase modulation. In this method, the receiver is required to extract data by performing a delay interference on each optical signal having the same polarized wave which is received every other symbol. In a general DPSK method, a differential encoding is performed to assign data to a phase difference of optical signals between two adjacent symbols. In contrast, in an Apol-DPSK method, a differential encoding circuit is required to perform a differential encoding between every two symbols.
While processing a differential encoding is accomplished by a high-speed digital circuit, the last output information on an optical signal phase has to be retained in a delay element to calculate a phase difference. This means that, in the differential encoding circuit, there is a feedback path which operates at a symbol rate. Since the bit rate in optical transmission is ultra-high, for instance, 40 Gbps or 100 Gbps, and the feedback path is required to operate at several tens GHz, it makes the differential encoding circuit extremely difficult to be implemented.
For dealing with the foregoing problem, there have been developed various differential encoding circuits operable at a high-rate. For example, Patent Document 1 mentioned below discloses a circuit-production technique that reduces an operation speed of a digital circuit by laying out a differential encoding circuit in parallel configuration, and thereby enables implementation of the digital circuit through a general LSI process.